Cellular telecommunications devices, such as those operating in accordance with the GSM, UMTS, HSPA, WiMAX or LTE Standards communicate wirelessly with base stations of a cellular telecommunications network using a cellular telecommunications bearer. Known mechanisms allow the mobile devices to move between the base stations without interruption of telecommunications services and without requiring user interaction. The radio spectrum used for cellular telecommunications is licensed for use by an appropriate governmental authority, such as Ofcom in the UK or the FCC in the USA. These mobile broadband networks have been very successful in delivering broadband services such as internet access, and in providing coverage for a very large proportion of the population and area of a country. As indicated above, these networks use licensed radio spectrum bands; however, the available bandwidth is very limited.
There are also technologies which use unlicensed radio spectrum bands such as WiFi or Bluetooth, but their range and coverage is very limited (they are used predominantly for indoor communications). WiFi mobile telecommunications devices communicate wirelessly with one or more WiFi access points. The radio spectrum used for WiFi communications uses a non cellular telecommunications bearer and is unlicensed by a governmental authority.
Further, there are non-cellular radio spectrum bands that are licensed to license-holders, which do not operate a service all the time everywhere in a country, such as TV stations or certain military applications. These license-holders are referred to herein as primary license-holders. So-called “white spaces” are parts of this non-cellular licensed spectrum which are unused in a certain geographical area for a certain time period.
Release of these underused white space frequency bands for other uses (without geographical or time limitations) would be highly advantageous. Services currently using these underused white space frequency bands could be moved to other frequencies. However, currently, there are no proposals to change the use of frequency spectrum in this way.
Regulators such as the FCC in the US and Ofcom in the UK have started to develop novel regulatory concepts for such white space frequency bands which try to make use of the spectrum whilst protecting the primary licence holder. Using white spaces efficiently, avoiding interference between different devices using white spaces and providing reliable, predictable services is technically very challenging. Several research projects and standardization bodies such as IEEE P.1900 and IEEE 802.22 have suggested technical concepts which use these white spaces, but these concepts have not been applied commercially since they have many technical difficulties.
The use of white space spectrum by communications devices is sometimes also called cognitive radio, as such devices have to be aware of their environment to determine whether they are allowed to transmit or not. There are two principle methods to determine this:                Geolocation database: The white space device consults a geolocation database to see whether it is allowed to use white space spectrum within a certain area for a certain period of time. The white space device has to determine its location with a certain accuracy (e.g. using GPS), and the database has to be maintained, e.g. by the regulator or a third party. Before a white space device can use the white space spectrum, it must reliably communicate with the geolocation database.        Cognition and sensing: The white space device measures the white space spectrum it wishes to use to establish if this spectrum is already used in its vicinity or not. This requires a very sensitive receiver which is expensive, or modification of the primary (licensed) user signal with beacons which is not very practicable. Other possibilities are cooperative sensing, where multiple white space devices work together to determine if the spectrum is occupied. Spectrum sensing takes a long time, and requires high power consumption.        
A difficulty with accurate sensing will briefly be described with reference to FIG. 1, and is the so-called hidden node problem. A primary (licensed) signal transmitter (and optionally also receiver) a, hereinafter referred to as a primary user access point, AP, is shown. An obstacle b is within the coverage area. Behind the obstacle b a cognitive radio device c measures the radio spectrum environment. At the device c the received signal strength of the primary user AP a is very low, because of the obstacle b between the primary user AP a and the cognitive device c. To be able to detect the low primary user signal, a sensitive radio is needed. If the cognitive radio device c is not capable of detecting the signal of the primary user AP a, it will then incorrectly determine that the spectrum is unused and may begin using this spectrum—thereby leading to interference at a primary receiver white space device d when it attempts to communicate with the primary user AP a.
This problem is especially severe in the uplink, i.e. the transmission of the white space device d (or cognitive radio device c) to the primary user AP a. Interference can be caused by the cognitive device c to white space device d. One example could be a laptop or smartphone with a white space radio (as cognitive radio device c), which could disrupt a TV receiver (as white space device in the same or a neighbouring room.
A communication system for operation in white space spectrum may use either paired (Frequency Division Duplex, FDD) or unpaired (Time Division Duplex, TDD) spectrum. Usage of paired spectrum would require a spectrum allocation by a regulator which would be difficult to obtain. Unpaired operation (TDD) may cause severe interference within the same white space channel and in neighbouring channels when the uplink from a white space device is colliding with the downlink of a close-by white space device either using the same communications system, or another communications system also using white space spectrum. Time synchronization of different devices of one or multiple systems could partially mitigate such interference issues, but not completely.
Some white space communications systems known in the state-of-the art use a so-called cognitive pilot channel to exchange information between white space APs and devices. However, this cognitive pilot channel needs to be agreed between different systems, needs to be available everywhere, and needs to be reliably found by all devices within the potential white space channels. This is incompatible with the concept of cognitive radio and white spaces, with dynamic allocation of spectrum. Also, the cognitive pilot channel would have to be standardized everywhere in the world in order to enable mass-market products, which is very unlikely.
Another issue with existing white space/cognitive radio devices is that the location information and sensing signal information is usually not very accurate, which requires that geographical exclusion zones are created around primary spectrum users, so no other use of the spectrum is allowed in that exclusion zone. Also, if a white space enabled mobile device is granted access to white space, the area over which access is exclusively granted needs to be made very large. These factors limit the efficient usage of white space spectrum in a certain country or area.
Bonding different carriers together, i.e. consecutive and non-consecutive carrier aggregation is known in principle for OFDM systems like LTE. However, these concepts operate in licensed bands and do not address white-space specifics. Also, they use usually only either FDD or TDD.